A traditional sailboat includes a hull, a sail-foil, and a keel-foil. The sail-foil travels through a first fluid (air) at a first vector velocity. The keel-foil travels through a second fluid (water) at a second vector velocity. The difference in vector energies is translated into work used for moving the boat. Both the sail-foil and the keel-foil have drag, and each individually is unable to compensate for its own drag. However, because the sail-foil and keel-foil are coupled through the hull of the boat, each compensates for the other's drag.
Conventional sailboats experience a torque effect called heeling. Heeling is caused by a downwind force operating on the sail compounded by an upwind force operating on the keel. On monohulls, catamarans, and trimarans, the keel-foil is located either directly under or nearly under the sail, resulting in a heeling torque. This heeling torque causes the boat to tilt to leeward, increasing hull drag, reducing sail efficiency, and, when large, causing the boat to capsize.
With reference to Prior Art FIGS. 1A-1C, heeling torque in monohulls (FIG. 1A), catamarans (FIG. 1B) and trimarans (FIG. 1C) is illustrated. In the monohull of FIG. 1A, a sail foil 23 and keel foil 22 are rigidly coupled to a hull 21 as shown. When a wind 20 is incident on the sail foil, a sail force F.sub.sail operates at or near the point of the center of effort of the sail foil 23 in the direction shown, and a keel force F.sub.keel operates at the center of effort of the keel foil 22 in the direction shown. F.sub.sail represents the total force of air on the boat, and F.sub.keel represents the total force of water on the boat, excluding buoyancy forces. These operating forces are offset by a distance d, generating a moment on the hull 22, referred to as a heeling torque .tau..sub.heel. Similarly, in the catamaran of FIG. 1B including dual hulls of pontoons 25A, 25B joined by connecting structure 24, and the trimaran of FIG. 1C including dual hulls of pontoons 25A, 25B and dual connecting structures 24A, 24B, the operative forces F.sub.sail, F.sub.keel generate heeling torques .tau..sub.heel in the respective crafts.
Ballast is employed to compensate for heeling torque in conventional sailboats. In smaller boats such as Sunfish.TM. and Lasers.TM., human ballasts are used. Sailors "hike out" on the windward side of the boat to level off the boat. Dual-hull catamarans employ the advantage of a longer moment arm between hulls or pontoons, increasing the metacentric height, thereby allowing for larger sails and increased speeds, but if the catamarans are small in size, they still require human ballast. In larger sailboats, the ballast weight of the sailors becomes insufficient. For this reason, weight is added to the keel of the boat, increasing the displacement and hull drag. In some monohulls, up to 70 percent of the weight of the boat is lead ballast in the keel.